Imaging X-ray devices, such as, for example, a C-arm X-ray apparatus or a computed tomography apparatus are increasingly being used for the clarification of medical questions. The use of X-rays exposes patients to increasing amounts of radiation so that an appropriate and dose-optimized usage as defined by the (“as low as reasonably achievable”) is necessary in each examination. Accordingly, the aim of medical imaging is to apply the lowest possible X-ray dose to a patient for the generation of one or more X-ray images.
Different measures are used to achieve this. One method for dose optimization is to adapt the X-ray tube current to individual X-ray attenuation properties of a patient.
To this end, modern X-ray image recording devices have automatic dose control systems which are used to adapt the X-ray tube current for an X-ray scan or an X-ray image recording automatically or semi-automatically to the X-ray attenuation properties of the patient. For the determination of a suitable tube current profile, it is advisable to find a middle way between the lowest possible radiation exposure and sufficient image quality to enable the medical question to be resolved. Consequently, an automatic dose control system results in a tube current profile for an X-ray image recording, which determines, in dependence on the X-ray attenuation distribution of a patient for each position along the patient's body axis and/or each direction of projection, the values for the tube current that produce the desired image quality. The term tube current modulation is also used in this context.
Known automatic dose control systems are based on topograms. A topogram corresponds to a conventional, two-dimensional X-ray superposition recording. It measures the individual X-ray attenuation distribution due to a patient in a specific direction of projection in which the X-rays pass through the patient and depicts this by way of different grey values. An automatic dose control system uses this X-ray attenuation to determine a suitable tube current profile or for tube current modulation. In a known method, before recording an X-ray image, one topogram is recorded of a patient in the lateral direction and one in the anterior-posterior direction and in each case the X-ray attenuation distribution is determined with reference to the grey-value distribution in the corresponding direction for the patient.
The decisive quality feature of an X-ray image recorded with the X-ray image recording device is the image noise. The image noise is defined as a desired value corresponding to the medical question and the automatic dose control system uses the reference values for tube current and X-ray attenuation corresponding to the desired image quality, which may be prespecified by a user or made available to him for selection, and the X-ray attenuation distribution derived from the topograms to determine an optimal tube current profile for the patient. In other words, the automatic dose control system uses the X-ray attenuation distribution of a patient for each position along the patient's body axis and/or each direction of projection to determine the values for the tube current which provide the desired image quality. Corresponding modulation of the tube current during the X-ray scans ensures that the desired image quality of the X-ray images is achieved with a reduced X-ray dose.